1. Field of the Invention
The present invention relates to a hydraulic power steering apparatus suitable for use in vehicles, and more particularly to a hydraulic power steering apparatus in which the flow rate of pressurized fluid supplied to an assist force generation mechanism is controlled by a flow control valve.
2. Discussion of the Prior Art
A hydraulic power steering apparatus is usually provided with a hydraulic pump and a flow control valve for supplying a pressurized fluid to an assist force generating mechanism at a constant flow rate. In such power steering system, the energy consumed by the hydraulic pump increases in accordance with an increase in the flow rate of the pressurized fluid. Therefore, the conventional hydraulic pump always consumes an amount of large energy or power.
In order to solve the above-mentioned problem, improved power steering apparatus have been proposed for reducing the flow rate of pressurized fluid during high speed traveling. Examples of such power steering apparatus are shown in Japanese Patent Publication No. 54-5571 and U.S. Pat. No. 4,714, 413. For example, in the apparatus of Japanese Patent Publication No. 54-5571, there is provided a vehicle speed sensor S, an amplifier A for amplifying a signal from the sensor S, and an electromagnetic valve SV which responds to the amplified speed signal, as shown in FIG. 1. The valve SV operates to reduce the pressure in the spring chamber of a flow control valve FC in accordance with an increase of the vehicle speed, thereby reducing the flow rate of the pressurized fluid supplied to the assist force generating mechanism which is composed of a rotary valve RV and a power cylinder PC. This system reduces the energy consumption of the hydraulic pump P. The power steering system also has a preferable characteristic that assist forces generated during high speed traveling are smaller that those during low speed traveling.
The conventional power steering apparatus, however, have a drawback that the flow rate of the pressurized fluid supplied to the assist force generating mechanism decreases when a steering wheel HD is turned. This causes a decrease of the assist force, giving a dull steering feel to a driver. Namely, when the driver steers the steering wheel, the rotary valve RV of the assist force generating mechanism operates to supply the pressurized fluid to a power cylinder PC for generating assist force, whereby the back pressure of the servo valve increases. This causes an increase of the flow rate of the fluid flowing through the electromagnetic valve SV, whereby the pressure in the spring chamber of the flow control valve FC decreases. This pressure decrease causes the flow control valve FC to increase the amount of the fluid draining to a reservoir R through a bypass passage BP, whereby the flow rate of the fluid supplied to the assist force generating mechanism is reduced.